Secret signaling



2 Sheets-Sheet 1 Fi ed Sept. 9, 1920 SECRET SIGNALING B. W. KENDALL Original July m 1926 Juily 2Q) 1926.

B. W. KENDALL SECRET SIGNALING Original Filed p 9. 1920 2 SheetsSheet 2 Patented July 26 I926.

BURTON W. KENDALL, OF EAST ORANGE, NEW JERSEY, ASSIGNOR T6 WESTERN ELEC- TRIG COMPANY, INCORP YORK.

RATED, OF NEW YORK, N. Y-, A CORPORATION OF NEW snomer sre uerme.

Application filed September 9, 1920, Serial No.

The present invention relates to the control of electrical waves for the transmission of signals or for other purposes. h

More particularly the invention relates to secret signaling and especially to secret telephone communication or to the secret transmisson of sound by electrical means.

Secret signaling by means of secret codes previously agreed upon by the communicat- 10 ing parties has long been used. In such systems the signal elements themselves are not necessarily concealed from outsiders, for

unless the code is known the signal elements cannot be assembled to make up the transmitted message. The transmission of the signal elementssuch as telegraphic letters or visual signs could therefore be carried on openly. I I In secret telephony, however, the message cannot readilybe decoded after reception,

but must be received as a restored and recognizable sound wave. This means that the receiving system must act automatically. to untangle the confused jumble purposely produced at the transmitter and to deliver intelligible sounds at the receiver. The types of apparatus available in a practical system for accomplishing the deciphering of confused currents are of limited variety and the order of secrecy obtained depends, of course,

upon the degree to which the'voice current elements that are transmitted can be conce'aled from the possibility of detection by an outsider who may beskilled in the use of this fairly familiar apparatus. The use of commutators, interrupters and similar apparatus for breaking up the speech still permits,

the transmission of actual speech current elements and it is conceivable that from the study of a phonographv record or other we ord of the waves transmitted the frequency of the commutatqr and the nature of thespeech mutilation used might be discovered. Other fairly simple methods of disguising 4 the voice are subject to similar limitations as to the degree of secrec It has been as modulations of a high frequency carrier wave, the carrier wave itself being suppressed and being continuously, varied in frequency. In order to receive the speech.

it is necessary to supply at the receiver a wave of the carrier frequency, and the possibihty of an outsider doing this is greatly inal frequency either of proposed to transmit speech 409,073. Renewed September so, 1925.

lessened by making the frequencyof the carrier wave vary continuously in a predetermined manner. Such a system is disclosed in the United States atent to W. E. Beatty, No. 1,454,532, dated ay 8, 1923. The system disclosed in that patent is particularly adapted to radio communication, but would have its limitations when applied to a line. For instance, when the system disclosed in that patent isused for secret radio communi cation it is not necessary to suppress currents of the .voice frequency at the output circuit of the transmitting modulator since the voice frequency waves would not be radiated and this fact somewhat simplifies the apparatus that would be necessary in a radio systhe balanced modulator used in the system referred to should preferably suppress both i the unmodulated carrier component and the voice currents completely, but if a filter is used to suppress the unmodulated carrier component, the variation of the carrier frequency is limited to a' comparatively. small range. The reason for this limitation in the range of variation of the carrier frequency in such a system is more fully pointedout in the application to V. L. Hartley, Serial No. 408,890, filed September 8, 1920.

It is a main object of the present invention to provide secret telephone transmission by modulated Waves of preferably variable frequency in which no componentsof, the origthe sound wave currents or the carrier wave find their way to the line, in which the waves transmitted to the line yield. only unintelligible signals by detection and in which the character of'the transmitted currents makes deciphering of the scheme of carrier frequency variation the apparatus at the cooperating distant station.

In'the system to be described a carrier wave of a varying frequency is modulated by the signal wave, the unmodulated component of the carrier wave is completely suppressed,giving a pure modulated wave, the signal wave is suppressed except as it appears as a modulation component of the carrier wave, and only one of the resulting side bands of the modulated carrier wave is trans- 'mitted. It is essential, of course, not to alsuppress one side band of the modulated carrier wave since if both side bands were transmitted a simple detector would yield currents of the double frequencies of the voice which are to a certain degree intelligible.

In Fig. 1, the line 1 is balanced by the usual network 2 and is provided with conjugate coils 3 and 4 to enable two-way communication thereover without interference between the transmitting and receivin circuits. The transmitting apparatus is s own in the upper portion of the figure and" the receiving apparatus is shown in the lower portion. The transmittin oscillator 5 generates a wave of preferably continuously varying frequency from which wave the carr1er wave is derived. The oscillator is of 'a well-known type employing a thermionic tube havmg a filamentary cathode, a grid and a plate or anode or equivalent electric discharge device and having a tuned circuit for; controlling the frequency of the generated wave. The tuning of this circuit is cont nuously varied and for purpose of illustratlon 1n the present case the capacity ele ment 6 of the tuned circuit is represented as being continuously varied b having one of Its plates rotatively driven y the motor 7. Connection is made from the plate of the oscillator to the fixed condenser-plate and the grid is connected to the movable plate of the condenser'through-the brush 8 which bears upon the shaft employed for rotating the condenser plate. This shaft is indicated as bemg made up of conducting and insulatmg portions shown as respectively white and black in the drawing in order to'isolate the condenser electrically from the rest of the apparatu By properly shaping the condenser plate or plates the frequency of the .oscillator can be made to vary cyclically in a desired manner. The oscillator is coupled by means of the coils 9 to the input circuit of the modulator M,,.

In the present system lt iS necessary, in

order to receive the transmitted signal to have at the receiver a. wave of the carrier frequency, which means that the supplied -wave at the receiver must vary in frequency independently driven motors for a very long time in step with each other a correcting system is preferably employed. The details of this correcting system form no part of the present invention and any known system which is suitable may be used in the system of the resent invention. However, the synchronlzing system indicated in the drawing is preferably that disclosed in the patent to R. A. Heising No. 1,501,711, dated July 15, 1924.

At each station the motor is made to drive a contact making mechanism. At the con trolling station these contacts cause the transmission through the system of synchronizing impulses and at the receiving station the contacts control the reception of these impulses by a local correcting mechanism. If the two driven devices are in step the correcting lagging or leadin with respect to the distant controlling evice the correcting 1mpulse causes a phase shift in the drlven device with respect toits driving motor to restore the device to the in-phase position.

impulses produce no result, but if the distant driven device is either The contact making mechanism at the transmitter consists of any desired number of conducting segments 11 all connected by a wire 12 to battery 13, and an inner con ducting ring 14 adapted to be periodically connected to the segments 11 by the rotating brush arm 15 driven mitting loop 16 and the opposite side-of the circuit 16 is connected to battery 13 so that each time the brush arm places ring 14 in connectiontwith-the segment 11 the battery 13 is closed across the circuit '16 and an impulse is transmitted to the line 1. These impulses are -merely phase-correcting 1m pulses and haveno sign canoe whateveras to by the motor 7. The ring 14 iscQnnected to one side of the transthe signals transmitted. Since thenumber of impulses transmitted per revolution of the condenser plate may be varied at'will,

as may be also the number of scallops on the condenser plate, the gear ratio, etc., these correcting impulses give no cluetp the manner of variation of the carrier frequency. Choke coils 18 and condenser 19 act 'as a low pass filter, permitting the transmission of the correcting impulses butkeeping the path through the commutator free of the energy of the high frequencies used'in transmitting the signals.

currents so transmitted are impressed upon 1 neeaeao The signals to be sent, assumed in the present case to be sounds and particularly speech, produce currents in the microphone circuit 20, shown as including a microphone and a battery. This circuit is coupled to the filtering circuit including the low pass filter 21 so designed as to transmit freely currents of the frequencies from zero to an upper limit preferably about 2200 cycles or at least suficiently high to include the essential frequencies comprised in the voice range, but to suppress higher and unessential harmonics. This and the other filters throughout the system may be designed in accordance with the principles set forth in the United States patent to G. A. Campbell 1,227,113 issued May 22, 1917. The essential voice frequency the input circuit of thethermionic modulator M to which is also supplied a high frequency Wave of frequency 3) from the source 22 which may be an oscillatifig tube or other suitable generator of sustained and constant high frequency 39. By the well-known ac,-

tiond of the modulator combination frequencies will be produced involvin p and a, where 8 represents the individual frequencies of the currents transmitted by filter 21. The most important of these combination frequencies will be p, 8, 2p, 28, p-f-c and 39-5. If p is chosen considerably above the highest frequency represented by s the only components of high frequency than p will be 2;!) and p-l-s. The component of frequency 72 and the other lower frequency currents and the component of frequency 2 may be suppressed by the band pass filter 23. The cur-v rents of p-l-s frequency together with the variable frequency current supplied from the oscillator 5 are impressed u on the input circuit of the modulator M The frequency of the current from oscillator 5 may be represented by p+c+al where p and '0 are fixed, but (i varies from zero to a fixed limit in accordance with a prearranged scheme. As explained above, this variation in the oscillator frequency is efl'ected by the variable capacity 6. The frequency '0 may be audible but is preferably above 2200 cycles as is also the frequency go. As a product of modulation of current of the frequency p-Fc-l-d by currents of the frequencies p+a there will result components of the frequencies involving such as p+v+d+p+c, etc, and one prominent component of frequency (p-l-c-l-d) (pd-cg =o+ds not involving the frequency 1). ince the frequency 72 may be made as high as desired, this side band of frequency 'v+ds can be made of much lower frequency than p-l-a, the lowest of the other frequency components of the output current of modulator M and so may easily be separated by the low a ass filter 2% which is so designed as to out i all frequencies in cluding p and those higher. The condensers It will be seen from the above description,

therefore, that none of the currents of signahng frequency 8 can net to the line on account of the band pass 'filter 23, and that no current components of the frequency ;0 or frequency p-I-v-l-d can get to the line on account of the filters 23 and 24 and that only one side band 0+d-"s can be transmitted. These filters may be designed with precision and may be made up of a large number of sections with the result that they accomplish for all practical purposes comp ete suppression of the undesired frequencies.

It is to be understood that any modulation or modification of a sinusoidal carrier wave produces acomplex resultant which is no longer a pure sinusoidal wave but at least at the initiation and termination of the modulating operation and, in general, throughout its duration includes sinusoidal components of one or more frequencies dif ferent from that of the unmodulated carrier wave. It is customary in the carrier wave transmission art to denote all components of a modulated wave of lower frequency than the unmodul-ated carrier wave by the term lower side band.and those of higher frequency as the upper side band. In the case of speech modulated waves, as is well known, each side band extends over a considerable frequency range. Tf,'on the other hand, the modulating wave be of a single,

frequency, that is sinusoidal, each of the resulting side bands will likewise be of a single frequency. lit is in this generic sense that the term side band is used in this specification and in the appended claims and it therefore is applied to, modulating operations of any kind whatever and irrespective of the nature of the modifying signal or agency.

Before considering the apparatus at the distant station on the line, the receiving portion of the circuit of Fig. 1 will be described. The receiving branch 26 coupled to the coils 3 and 4 contains the filtering circuit 27 which may be designed to suppress currents of all frequencies except those used 1n the one channel indicated. If the line 1 is not composited or multiplexed the filter 27 may be dispensed with. It is assumed in the present case that the same synchronized driving devices control the variation of the carrier current used in transmitting'in both directions over the line. This does not necessitate the use of the same frequencies for transmission in both directions, however, nor v represented by v-ld-s.

, "frequencies combine in the detector in accord-ance with the well-known homodyne method of U. S. Patent 1,330,471 granted February 10, 1920, to B. W. Kendall, to. give a resultant current having a component of frequency 8 and the current of this frequency is selected from the higher frequency components by the low pass filter 31 and is received in the telephone receiver 32. The amplifier A in addition to amplifying the received wave acts as a one-way transmissioh element to prevent the wave from the oscillator 29 from passing to line 1.

In Fig. 2 the line 1 is shown as having a line balance as in Fig. 1, a transmitting branch 33 and a receiving branch 34. Synchronizing impulses received from the line 1 pass selectively through the filtering circuit 35 to the correcting mechanism indicated at 36. This correcting mechanism is' located between the driving motor and the driven portion of theshaft controlling the condensers 6 and 28 and, as explained, shifts the phase of this shaft relative to the motor shaft when necessary in order to bring the driven members at the intercommunicating stations into step. Since, as above stated, the details of this correcting mechanism may be of known character and the mechanism is preferably of the type clearly disclosed in the Heising application above referred to,

but forms no part of this invention, further details are not deemed necessary to the present' disclosure.

The wave received from the station in Fig. 1 and having a component of frequency o+d-s is selectively transmitted from the filter 37 through amplifier A to the detector D which is also supplied with a wave of variable frequency v-l-d from the oscillator 0 the frequency of which is controlled by condenser 28. The signal component of frequency 8 yielded by the action of the detector is selectively passed by the receiving filter FR'and produces an audible signal in the receiver 38.

The station in Fig. 2 is illustrated as equipped with transmitting apparatus which may be similar in every respect to that illustrated anddescribed in connection with Fig.

1. The oscillator O, has its frequency varied by condenser 6' in a manner corresponding to the variations of the distant receiving ponent of frequency o+ds transmitted over the line may be audible or inaudible depending entirely upon the frequency of 12+d. This current may be of the frequencies used for carrier waves in radio or carrier current transmission or it may be of about the upper limit of the essential speech frequencies. In the latter case, the band represented by the components of frequency o+d-s will occupy substantially the same range as the original speech currents' but will be unintelligible.

Although the method described for combining and utilizing the waves of different frequencies is preferred, there are obviously other ways of arriving at the same result. For instance, if instead of selecting the band including frequency p-l-s from the modulated wave in the output circuit of the first modulator, the lower side band of frequency 10-8 is selected, and impressed on the input circuit of the second modulator together with a wave of varying frequency p- (o-l-d) the side band of frequency o|-ds of the,resultin'g modulated wave will still be obtained for transmission. But if components of frequency [2+8 and p (0+0?) are supplied to the second modulator, the resulting band of frequency 'v+d+s will be obtained for transmission and as in theother case this same band o+d+s may be obtained by combining waves of frequencies 72 8 and p-{ -v-i-d. Although the invention has been disclosed as embodied in a conductive transmission system, the principles of the invention are obviously applicable as well to radio transmlssion. The system is also capable of variation in other respects from the precise form that has been illustrated and described and the invention is not to be construed as limited to the specific circuit arrangements muted band and transmitting to the distantstation only one side band of the secondv modulated wave, the frequency of one of said waves being continuously varied.

2. The method of secretly transmitting signals consisting of a plurality of frequency components which comprises modulating a wave in accordance with said signals, producing a wave of varying frequency, modulating said varying frequency wave by one side band only of the signalmodulated wave and transmitting one side lll a by the (Bid band only of the frequencies resulting from the modulation of said, varying frequency Wave.

3. The method of secret signaling which comprises modulating by currents of signaling frequencies a wave of varying frequency, suppressing all the unmodified components of both said varying frequency and said signaling frequency waves and transmitting only one side band of the resulting modulated variable frequency wave, supplying a wave at the receiver, transmitting discontinuous control impulses to the receiv ing point to'cause the Wave supplied thereto to vary in frequency at all times substantially in the same manner as that of said varying frequency wave and combining the transmitted side band with the wave generated at the receiver to reproduce the signaling current.

4. The method of deriving waves for secret transmission of signals having a range of frequency components, comprising modulating a wave in accordance with said signals, modulating a varying frequency wave by the signal-modulated wave, thereby producing waves of .frequencies equal respectively to the sum and the difierence of said varying frequency ahd said signalmodulated wave frequency and each modulated by the signal, and selecting one of the modulated waves thus produced from the other waves.

5. The method of transmitting a pure modulated wave which comprises modulating a wave in accordance with variations to be transmitted ofiupying a range of frequencies, selectively transmitting resulting wave components of the upper side frequencies only, combining said upper side frequency components with a varying frequency wave of a difl'erent order of frequency to produce a modulated varying frequency wave having frequencies represented difierence between the frequencies of the waves so combined, and transmitting only the lower side frequency components of the modulated wave so produced.

6. The method of secret signaling which comprises reducing a pure-modulated carrier wave a'ving a cyclically varying frequency, transmitting one side band compo-" nent of the wave so modulated but suppressing transmission of currents of all other frequencies, producing at the receiver a cyclically'varying wave similar in frequency to the component of the wave transmitted, transmitting independently. of the waves used in signal transmission control impulses for maintaining in stepthe frequency-variations in the transmitted wave and the said wave generated at the receiver and combining said transmitted component with the wave produced at the receiver to reproduce the signal.

7. In a system of secret signaling, means for modulating a high frequency accordance with signals having a plurality of component frequencies and for deriving as a component a single side band of frequencies of the modulated wave. means for generating a wave of varying frequency and for modulating said wave by the single side band from said first means and means for transmitting a single side band of the frequencies resulting from the action of said second means and for preventing transmission 'of components of other frequencies.

8. In a system of secret signaling, means for generating a wave of fixed frequency and a wave of varying frequency, means for modulating one of said waves in accordance with signals having a plurality of component frequencies and for modulating the other of said waves in accordance with the signal-modulated Wave so produced, and means .for selectively transmitting one side band of the frequency resulting from the second modulating operation and for suppressing the transmission of currents of the other frequencies.

9. In asecret signaling system, a source of current of sustained frequency, means for modulating said currents in accordance with signals having a plurality of component frequencies, a filter for transmitting only one slde band of the frequency components comprised in the modulated wave, a source of waves of varying frequency and means for modulating said waves of varying frequency in accordance with the currents comprised wave in in said side band of frequencies to produce frequency bands representing the sum and diderence frequencies of said varying frequency waves and said side band frequencies and a filter for transmitting one of said frequency bands only and for suppressing transmission of the other frequency components. I

10. In a secret signaling system, statlons, a generator of high frequency waves at each station, a driven control member for each of said generators for introducin s1m1lar variations in the character of t e generated waves and independent driving members at the respective stations for said control members, means for maintaining said separately driven control members in step, means at one station for modulating a enerated wave in accordance with SlgllfilS avmg a plurality of component frequencies and for tem comprising means for producing an unmodulated wave, means connected thereto for modulating said wave, a filter for selectively passing frequencies lying to one side of said unmodulated wave frequency but preventing the transmission of said unmodlilated wave and currents of all frequencies 0 the opposite side of said unmodulated wave frequency, means for modulating a high variable frequency wave in accordance with current of the frequencies passed by said filter and a filter for selecting for transmission onl those currents comprised in a single side and resulting from the modulation of said high variable frequency wave which lacks the component representing either of said different high frequencies.

12. In combination, a modulator for modulating a high fre uency wave in accordance with speech, clrcuits for impressing a wave of said high frequenc and currents of speech frequency upon sai modulator, and a low pass filter in the speech current circult for transmitting tosaid modulator currents of essential speech frequencies but for suppressing the higher harmonics'of speech.

13. The method of suppressing the unmodulated component of a varying frequency earner wave, which comprises producing a modulating wave, increasing the effective frequency of said modulating wave by a fixed amount, reducing a varying frequency wave, modu ating sald varying frequency wave by said increased frequency modulated wave, and selectin for transmission from the resulting modu ated wave a modulated component, to the exclusion of any unmodulat component of varying frequency. 14 In modulation of varying frequency carr1er waves, the method of suppressing transrmssmnof an unmodulated' component ofsaid varymg frequency which comprises producmg a modulating wave, increasing the frequency of said modulating wave, producing a varymg frequency wave, modulatlng said wave of varying frequency by sa1d 1ncreased fre uency modulating wave o yleld modulate and unmodulated components having wide frequency separation, and selecting out from the result nt m du- 'aeeaeao lated wave a wave having no unmodu'lated component.

15. A system for suppressing the'unmod ulated component of a varying frequency carrier wave, which comprises means for producing a modulating wave, means for increasing the effective frequency of said modulating wave by a fixed amount, means for producing a carrier wave of varying frequency, means for modulating said carrier wave by said increased frequency wave, and means for selecting out from the resultant modulated wave and transmitting to the exclusion of unmodulated components thereof a wave having no unmodulated component.

16. A means for producing a varying frequency carrier wave, means for producing a modulating wave,.means for lncreasing the frequency of said modulating wave, means for modulating said carrier' wave by said increased frequency wave to yield modulated and unmodulated components having wide frequency separation, andmeans for selecting out from the resultant modulated wave and transmitting to the exclusion of unmodulated components thereof a wave having no unmodulated component.

transmission system comprising till 17. In a carrier telephone system, a line circuit, a carrier transmitting. circuit including a vacuum tube modulator connected to said line, a speech transmission circuit connected to said modulator, a low pass filter in said speech transmission circuit for restricting the range of frequencies impressed on said modulator to the lower essential range of speech frequencies, a source of carrier waves J connected to the input of said modulator and a second filter between the modulator and the line for suppressing one side band of the modulated wave and selectively transmitting to line a range of frequencies including the other side band.

18. The method which comprises inverting speech waves to produce an unintelligible inverted audio-frequency band of waves and subjecting the various components of said inverted frequency band to a cyclic frequency variation.

19. A signaling system comprising means for producing speech waves, means for inverting said waves to produce an unintelligible inverted audio-frequency band of waves, and means for subjecting the various components of said inverted frequency band to a cyclic frequency variation.

20. The method of inverting the frequency cy hl her than 21113 ping th ineeaeao side-band and the unmodulated component of said wave of higher frequency, modulating asecond wave by the selected side-band. and selecting from the Wave so modulated the side-band having a frequency order 'the inverse of that of the original band, while suppressing the other side-band and the un modulated component of said second wave.

21. The method of inverting the frequency order of a band of frequency components to secure an inverted frequency band overlape frequency range of the original band, comprising modulating a wave of a frequency as high as'the highest frequency of the original band in,accordancewith the original band of components, filtering the wave so modulated to'pass only one sideband, modulating a second wave by said side-band, said second wave being of such frequency relative to the first wave as to cause the resulting side-band in which the frequency order is the inverse of that of the original band to overlap the frequency range of the original band, and selecting said inverse frequency side-band.

22. The method of inverting the frequency order of a band of frequency components to secure an inverted frequency band overlapband, comprising modulating a wave of a frequency higher than the highest frequency of the original band in accordance with the original band ofcomponents, filtering the waves so modulated to pass only the frer quency components of higher frequency than the highest component of the original band whereby transmission of all components of the original band is suppressed, modulating a second Wave by the components so passed, said second wave being of such frequency relative to the first wave as to cause the resultant side-band in which the frequency or der is the inverse of that of the original band to overlap the frequency range of the original band, and selecting said inverse frequency side-band.

In a signaling system, a source of waves of a band of frequency components, a source of waves of a single frequency, a

plug the frequency range of the original from transmission said wave of single frequency and components of lower frequency,

means torselectively transmit from the resultant modulated waves frequency components higher than the highest frequency of said band, a second source of waves of a single frequency, a second modulator for modulating said waves from said second source in accordance with the frequency components passed by said selective means, and a low passfilter for selecting from the resultant modulated wave in the output of said second modulator a band of frequency components having a frequency order the inverse of the original band of frequency components and for suppressing the other components of said resultant modulated Wave. 24. In, combination, means to generate a wave having a band of frequency components, means to generate a wave of a single frequency as high as the highest fre lency in said band of frequency components, means to modulate said single frequency by said band of frequency components, means to filter the resultant modulated wave so as to pass only frequencies higher than the highest frequency in said band, means to generate a second single frequency Wave,

modulating said Waves of sinmeans to modulate said second single frequency Wave in accordance with the frequencies passed by said filtering means, said second single frequencywave having such a frequency relative to the first-mentioned single frequency wave as to cause the production in a second-mentioned modulating process of a band of frequency, components having a frequency order the inverse of the original band and overlapping the frequency range of said original band, and means to select said inverse overlapping band of frequency components.

' In witness whereof, I hereunto subscribe my name this 7th day of September A. D., 1920. 

